"This category [science fiction] excludes rocket ships that make U-turns, serpent men of Neptune that lust after human maidens, and stories by authors who flunked their Boy Scout merit badge tests in descriptive astronomy."
- Robert Heinlein

It covers almost the entire body, provides cooling and ensures that almost no moisture is lost.

This is yet another of the fascinating inventions of Dune, a desert planet with no bodies of surface water at all. In this desiccated environment, even moisture is precious. Hence, the stillsuit, which reclaims the moisture lost by the body during the course of the day.

It is a very clever idea, but as far as I can tell, there are no efforts to take advantage of the basic idea. (Water being plentiful here on planet Earth.)

However, there is a lot of interest in the idea of recovering power from the human stride. Engineers who are interested in powering wearable computers have investigated the idea; check out Parasitic Power Harvesting in Shoes by some guys back in the lab at MIT. The study mentions that

"In previous studies it has been calculated that up to 67 Watts of power are available from heel strikes during a brisk walk (68 kilogram person, 2 steps/second, heel moving 5 cm.) This level of power extraction from walking would certainly interfere greatly with one's gait."

It's basically a micro-sandwich; a high-efficiency filter and heat-exchange system. The skin-contact layer is porous. Perspiration passes through it, having cooled the body. Motions of the body, especially breathing, and some osmotic action provide the pumping force. With a Fremen suit in good working order, you won't lose more than a thimbleful of moisture a day - even if you're caught in the Great Erg.

Here are some other quotes from Dune that illustrate different aspects of the stillsuit:

"...they stink to heaven in any closed space. It's from those suits they wear -- call them 'stillsuits' -- that reclaim the body's own water..."

...When he had tightened the chest to gain maximum pumping action from the motion of breathing, he had known what he did and why. When he had fitted the neck and forehead tabs tightly, he had known it was to prevent friction blisters...

...He found his stillsuit's watertube in its clip at his neck, drew a warm swallow into his mouth, and he thought that here he truly began an Arrakeen existence--living on reclaimed moisture from his own breath and body. It was flat and tasteless water, but it soothed his throat...

...Paul crouched at the sphincter, his stillsuit hood adjusted for the open desert--forehead capped, mouth filter in place, nose plugs adjusted. Only his dark eyes were visible: a narrow band of face that turned once toward her and away...

...I must remember to fasten the boots slip-fashion to help my stillsuit's pumping action, she thought...

Although it conjures up an odd image, water was held so dear on Dune that even domesticated animals needed help:

The concern on Arrakis was not with water, but with moisture. Pets were almost unknown, stock animals rare. Some smugglers employed the domesticated desert ass, the kulon, but the water price was high even when the beasts were fitted with modified stillsuits.

Finally, here's one more quote from Dune Messiah, the first sequel to the original novel.

...Paul sat on the edge of his bed and began stripping off his desert boots. They smelled rancid from the lubricant which eased the action of the heel-powered pumps that drove his stillsuit...

It's interesting to note that the matter of "gait" is very important to Fremen, since repetitive motion creates vibration that attracts the gigantic sandworms of Dune.

The sophisticated stillsuits hint that the Fremen, a culture that prefers not to participate in the feudal culture of Dune, are not merely simple tribespeople.

Compare to the transkin from Parasite Planet (1935) by Stanley Weinbaum.